The present invention relates to a manufacturing method of a display device such as a liquid crystal display panel, more particularly to an adjusting technique of a gap width between two device substrates.
As an image display device for a personal computer or other various monitors, a liquid crystal display device has been remarkably widespread. Such a liquid crystal display device is generally constructed in such a manner that a backlight as a planar light source for illumination is disposed on a back of a liquid crystal display panel to irradiate a liquid crystal surface of a specified expanse so as to even the brightness thereon, and that an image formed on the liquid crystal surface can be thereby visualized.
The liquid crystal display device includes a liquid crystal display panel constructed by sealing a liquid crystal material between two glass substrates, and a printed circuit board for driving the liquid crystal material. The printed circuit board is mounted on the liquid crystal display panel. A backlight unit is disposed on the back of the liquid crystal display panel with a liquid crystal display panel holding frame being interposed therebetween. An outer frame surrounds the above-described components. The glass substrate comprises the liquid crystal display panel and the electrodes for driving the liquid crystal material on which are formed.
A manufacturing process of a thin film transistor (TFT) liquid crystal display device among such liquid crystal display devices includes the steps of having an array step, a cell step and a module step.
The array step includes installing TFTs, pixel electrodes, data lines, gate lines and the like on one of the two glass substrates. Since the TFTs are regularly arrayed on the glass substrate, the step is referred to as an array step.
The cell step includes adhering the array substrate obtained in the array step and a color filter substrate as a substrate opposite to the array substrate, and injecting a liquid crystal material in a gap as a space between the two substrates, then sealing the gap. The substrate obtained in this step is referred to as a panel substrate.
The module step includes mounting an electric circuit for electrically controlling the panel substrate obtained in the cell step, and further mounting parts such as a backlight and the like.
The contents of the cell step among the foregoing steps will be described below. Sealant is coated on the periphery of the color filter substrate. The sealant is provided for sealing the liquid crystal material between the array substrate and the color filter substrate. Note that the sealant is coated in the form of a picture frame except a portion serving as an injection port for injecting the liquid crystal material. An example of such a sealant may include a thermosetting resin or an ultraviolet-setting resin. Meanwhile, an orientation film is formed on the array substrate, and then spacers are sprinkled thereon. The spacers are sprinkled for regulating a gap width between the array substrate and the color filter substrate. Examples of such spacers may include spherical silica, spherical polyethylene or the like. Note that, instead of sprinkling the spherical spacers, a technique of installing columnar spacers on the array substrate or the color filter substrate has been examined recently. After sprinkling the spacers, the color filter substrate is laminated onto the array substrate, and the sealant is cured. After curing the sealant, the liquid crystal material is injected-into the gap between the array substrate and the color filter substrate. For the injection, an injection apparatus using an vacuum injection method is used in many cases. Specifically, the laminated substrates are disposed in a vacuum chamber, and the gap between the two substrates is evacuated. The foregoing injection port is immersed in the liquid crystal material while maintaining the evacuated state, then, by returning the chamber pressure from the vacuum to the atmospheric pressure, the liquid crystal material is filled in the gap. Thereafter, the injection port is sealed by the thermosetting resin or the ultraviolet-setting resin. Thus summarizing the cell step.
The gap between the array substrate and the color filter substrate, that is, the cell gap, is extremely narrow. Typically the gap is 5 micrometers or less, desirably 3 to 5 micrometers. Image quality of the liquid crystal display device is greatly affected by the gap. Hence, the cell gap must be strictly controlled.
In order to control the cell gap, an even pressure must be applied to the substrates when the color filter substrate is laminated onto the array substrate. For this purpose, there have been various proposals including the ones disclosed in the gazettes of Japanese Patent Laid-Open No. Sho 57 (1982)-188018, Japanese Patent Publication No. Hei 6 (1994)-16137 and Japanese Patent No. 2976925.
In the gazettes of Japanese Patent Laid-Open No. Sho 57 (1982)-188018 and Japanese Patent Publication No. Hei 6 (1994)-16137, two methods have been proposed. In both of the methods, as shown in FIGS. 13 and 14, a laminated body composed of an array substrate 101 and a color substrate 102 whose gap has not been filled with the liquid crystal material yet is disposed in a sealed container 100 formed of an elastic material, for example, silicon rubber. Thereafter, as shown in FIG. 13, the sealed container 100 is decompressed, and alternatively, as shown in FIG. 14, an air pressure is applied from the outside of the sealed container 100. Since each of these methods is a method of applying a pressure to the substrates by allowing a fluid pressure to act on the elastic material, the pressure applied to the substrates is sufficiently even. Moreover, in the gazette of Japanese Patent No. 2976925, a laminated body composed of an array substrate and a color filter substrate is disposed in a sealed container formed of an elastic material. Then, the container is decompressed, and thereafter, such decompression is released to inject a liquid crystal material into a cell gap by making use of a difference between pressures of the inside and outside of the container, which is generated when the container is released to the atmospheric pressure.
However, among the techniques disclosed in the gazettes of Japanese Patent Laid-Open No. Sho 57 (1982)-188018, Japanese Patent Publication No. Hei 6 (1994)-16137 and Japanese Patent No. 2976925, in the method of decompressing the sealed container, the difference between the air pressures of the inside of the sealed container and the outside thereof (atmosphere) cannot be set at 1 atm. or higher. Hence, only a pressurization force up to 1 kgf/square cm can be applied to the array substrate and the color filter substrate. Here, the sealant is coated to have a thickness thicker than the cell gap, and the sprinkled spacers are overlapped from one to another. Thus, the cell gap with a desired width is hard to obtain. Although described later in detail, with such a pressurization force of about 1 kgf/square cm, the cell gap of a desired width was not able to be obtained. Accordingly, by the conventional method of decompressing the sealed container, it is difficult to adjust the cell gap to a desired micro value. Meanwhile, in accordance with the method of pressurizing the sealed container from the outside, it is possible to apply a pressure higher than that by the method of decompressing the sealed container. However, when the sealed container is released to the atmospheric pressure after the container is pressurized from the outside, a force acts in the direction where the array substrate and the color filter substrate are torn off from each other. Specifically, when a high air pressure is applied to the sealed container from the outside, the air in the sealed container is compressed so as to oppose such a pressurization force, resulting in a pressure increase in the cell gap. When the sealed container is released to the atmospheric pressure from the pressure-increased state, the pressure in the cell gap exceeds the air pressure of the outside, that is, 1 atm. in the atmosphere. Thus, the force acts in the direction where the array substrate and the color filter substrate are torn off from each other as described above, affecting adversely the control of the cell gap. Moreover, the force involves the possibility of exfoliating the sealant and one of the array substrate and the color filter substrate from each other. Such exfoliation in an uncured state of the sealant is not desirable since the exfoliation causes bubble mixing to the sealant and leads to the lowering of joining forces of the cured sealant.